AMPHIBOLE ZONING IN THE GARLAND-PEAK SYENITE, RED-HILL COMPLEX, NEW-HAMPSHIRE - CAMPTONITIC PARENTAL MAGMAS AND DIFFERENTIATION TO SILICA-OVERSATURATED SYENITES

The Garland Peak Syenite (GPS) of the Red Hill complex, New Hampshire, consists predominantly of amphibole, oligoclase, perthite, and quartz ; amphiboles have homogeneous kaersutite cores with strongly zoned rim s ranging in composition from pargasite to hastingsite to hornblende. The thin section scale association of kaersutite, an amphibole that ty pically crystallizes in silica-undersaturated magmas, with quartz sugg ests that the GPS magma experienced substantial changes in magmatic co mposition, including silica activity, during its crystallization histo ry. Kaersutite-bearing camptonites are also associated with the Red Hi ll complex. The camptonite amphiboles are very similar in composition to the core kaersutites in the GPS, suggesting that the earliest GPS l iquid may have had camptonitic affinities. In order to elucidate the p rocess whereby silica-undersaturated magmas differentiate to saturatio n, amphiboles in these rocks were analyzed by electron and ion micropr obe techniques. Amphiboles show a progressive increase in REE abundanc es from the camptonites to the GPS kaersutite cores to the GPS pargasi te/hastingsite/hornblende rims. The systematic change in REE concentra tions, and the variations in V, Ti, Sr versus Zr, Eu/Eu-degrees and La /Yb versus Ce, suggest a possible differentiation relationship for the amphiboles and imply that the GPS was derived from magmas similar to camptonites. Rimward depletions in Sr, Ti, V, and Eu/Eu-degrees, and t he increase in La/Yb values suggest that parental camptonites fraction ated plagioclase, magnetite, and amphibole to produce the silica-overs aturated GPS. Bulk-rock modelling agrees with the trace element record preserved in the amphiboles, that plagioclase, magnetite, and amphibo le fractionation caused silica saturation. Minor pegmatitic patches oc cur in the GPS. Ferrohornblendes in the pegmatites have REE abundances distinct from the other GPS amphiboles, and this difference may be du e to open system processes.